The present invention relates to an implantable apparatus for giving electrical stimulation to a body tissue, and especially relates to a vision regeneration assist apparatus for regenerating patient' vision.
A vision regeneration assist apparatus has recently been proposed as one of approaches toward treating blindness. Specifically, a device having a plurality of electrodes is implanted in an eyeball, etc. Electrical stimulation is applied to cells forming a retina, thereby regenerating patient's vision. Such a vision regeneration assist apparatus includes an internal body device that is equipped with an electrode for applying electrical stimulation to the cells forming the retina and a control unit (an integrated circuit) having a multiplexer function for controlling the electrode.
As an implantable apparatus for giving an electrical stimulation to a body tissue, an artificial middle ear for transmitting a sound wave to an auditory ossicle of a patient, a cardiac pacemaker implemented in a chest region of a patient for giving an electrical stipulation to a heart to suppress an occurrence of arrhythmic heartbeat, etc. are known.
If a body fluid invades an electronic circuit, such as an integrated circuit, provided in the internal body device, detrimental effects will be inflicted on circuit function. For this reason, device is made to protect the electronic circuit. For instance, the electronic circuit is placed on a mount made of an insulating material, such as ceramics, that exhibits superior biocompatibility; and is sealed with a metal case formed from a biocompatible, hermetic material [US 2008058896 A1 (JP-A-2008-55000)].
The electrodes provided on a substrate and the electronic circuit are electrically interconnected by wires formed from a conductive material filled in through holes of the mount. After having been filled into the through holes of the mount without clearance, such a conductive material is sintered. However, if a difference exists between the mount and the conductive material in terms of a coefficient of thermal expansion during sintering operation, distortion or cracks will become likely to arise in a surface of the mount, which will in turn affect placement of the electronic circuit. Accordingly, a material, such as silver, tungsten, or molybdenum, mixed with a predetermined additive in order to provide consistency between the mount and the conductive material in terms of a coefficient of thermal expansion has generally been used as a conductive material (see JP-A-2001-15869).
However, biocompatibility of tungsten or molybdenum (and an additive) has not been verified; therefore, it is difficult to adopt tungsten or molybdenum in consideration of a case where a body fluid invades a boundary (a clearance) between the substrate and the mount. Further, a conceivable approach is to use a material exhibiting superior biocompatibility, such as gold and platinum, for the conductive material. However, a great difference exists between the mount and the material in terms of a coefficient of thermal expansion, and a problem of cracking, distortion, etc., becomes likely to occur in the mount during sintering operation.